The present invention relates to sequence cameras for recording a series of images on a strip of photographic film.
The growth in the number of branch banks, all-night grocery stores and gas stations, and similar targets for robbery and pilferage have increased the need for reliable and inexpensive surveillance cameras.
This requirement encourages designs without the extensive features and performance of a typical "movie" camera. In particular, the rate at which the pictures are taken, or frame rate, need not be nearly as great. Frame rates of only 2 per second are typically adequate. In addition, a surveillance camera often need run only for a few minutes to capture all the events of interest. Mounted on a wall or ceiling, a surveillance camera need not be designed for small size or portability. Also, commercial electrical power is typically available to drive the camera.
A surveillance camera need not have a leaf or other complicated form of shutter. Because of the lack of serious size constraints, a simple rotating shutter may be employed. The typical rotating shutter is simply a disk with a pie-shaped cutout that is rotated between the film aperture and the camera lens. As the cutout passes the aperture, the film is exposed. In general, the exposure time is determined by the combination of the speed of rotation and the size of the cutout.
Of course, it is necessary to advance the film to the next picture location, or frame, prior to the next time the cutout rotates by the film aperture. Using a rotating shutter, the problem is to build an equally simple means to advance the film, appropriately synchronized to the shutter.
Separate motors for the shutter and film advance have been used. This not only necessitates the expense of two motors, but also requires complicated means to keep the two drive systems in synchronization. Also, a single motor can be employed and some form of clutching mechanism used to intermittently advance the film, while the shutter continuously rotates. This method still requires a complicated synchronization method.
The synchronization problem can be avoided by use of a common drive motor and positive, versus slipping or clutching, drive elements. The drawback has been increased mechanical complexity to provide the necessary intermittent film advance in conjunction with the continuous rotation of the shutter while ensuring a fixed or positive relationship between the shutter rotation and the film advance. Film advance mechanisms have been built using complicated pawl systems and complicated escapement gear trains.